In pulse-echo ultrasound investigations, the velocity of ultrasound scatterers can be estimated from the relative time delay between signals in successive A-lines. This relative time delay can be determined from the value of a time lag which maximizes the cross-correlation of signals from two successive A-lines.
If the ultrasound scatterers have low velocity, the relative time delay between the signals in successive A-lines may be much shorter than the pulse repetition rate of the ultrasound transmitter (that is: the time interval between signal samples). Conventional discrete time correlators only generate correlation estimates for time lags which are multiples of the signal sample interval. Thus, when the scatterer velocity is low, it is difficult to accurately locate the maximum of the cross-correlation function from cross-correlation samples that are spaced at multiples of the sample interval. This problem is particularly severe if the cross-correlation estimates are computed with one-bit correlators.